When selecting a high efficiency particulate air filtration solution, what factors need to be considered to optimize turbine efficiency, reduce maintenance frequency and give a better overall return on investment for a particular installation? Steve Hiner has some answers.
A comparison of two gas turbines operating side-by-side at the same site shows the impact filtration can have
Gas turbine inlet filter performance directly impacts the efficiency of the turbine plant in terms of power output and heat rate loss.
Turbines consume vast amounts of air and an effective inlet filtration system is vital to maintaining optimum performance and reducing the need for maintenance shutdowns. Ineffective filtration solutions will lead to problems in turbine performance that will require operator intervention. The level of impact will depend upon environmental conditions on-site and this means that a filtration solution needs to be designed with an understanding of the local installation conditions. Depending where the turbine is installed, the filtration system may need to handle sand, salt, dust, hydrocarbons, moisture or even snow and ice.
Fine particles entering a turbine can stick to turbine blades. This creates fouling which, as it builds up, affects the turbine’s aerodynamic performance. As output power reduces and heat rate rises, the operator will need to take the turbine offline to wash the compressor and restore its performance. The reduced turbine efficiency and the lost production time when it is taken offline have large cost impacts in terms of lost MW output and more fuel burned.
High efficiency particulate air (HEPA) filters offer greater levels of protection and so, in theory, will help to improve plant performance. However, the use of finer filters also presents challenges in terms of filter life. It has been shown that standard filter efficiency ratings from standardized laboratory-based filter testing do not necessarily result in the same performance when operated in the real world. Moisture and hydrocarbons found at site can cause sudden blockages that may result in pressure loss spikes and equipment downtime or damage.
Filter solutions may generally use either microfibre glass (glass fibre) or membrane media, typically ePTFE, to achieve higher HEPA efficiency. Microfibre glass has historically been the more popular choice in industrial environments as it offers robust and predictable filtration performance. ePTFE membrane technology is relatively new to this application, so how can operators determine which is best for their installation?
Both microfibre glass and ePTFE media types can achieve HEPA level performance with (H)EPA ratings of E11 or E12 to the international standard EN1822. Fundamentally, a higher filter efficiency requires finer media to filter out smaller particulates.
One of the differences between microfibre glass and ePTFE is the thickness of the media used to achieve this. Microfibre glass filters use a deeper filtration layer whereas membrane solutions use a single, very thin layer. The problem is that the reduced total filtration volume means membrane technology can be more sensitive to blockage from moisture and hydrocarbons.
Hydrocarbons can find their way into a turbine inlet as the result of many processes, including the by-products of combustion such as soot or unburnt fuel, oil vapour from lube oil vents, and general atmospheric pollution. When combined with moisture they can cause filters to block and, if the filter is not designed to handle this, blockages may occur very quickly and without reasonable warning. As the turbine strives to suck the air it needs through its inlet, the filter blockage causes an increase in pressure drop. Sudden pressure loss spikes can result in costly unscheduled downtime or even damage to machinery or ductwork.
ePTFE membranes have a thin, two-dimensional layer of high efficiency media which can quickly become blocked by moisture droplets. This media has been shown to be prone to sudden blockages and high differential pressure spikes. Plugging has been seen to occur anywhere from as little as three weeks after installation and requires urgent remedial maintenance to prevent wider system damage. It is this unpredictability which means further research and development into this type of technology is required to make it more robust for heavy industrial applications.
Microfibre glass media is around ten times thicker than a typical ePTFE membrane. Its greater pore volume makes it naturally less prone to sudden blockages, while its depth maintains an equivalent high particulate efficiency. Any pressure loss increase across the system has been proven to happen much more slowly than with ePTFE, giving the operator plenty of time to take action and making this type of media much more predictable in its performance.
Fibreglass is able to handle moisture better than membrane due to its thickness
The difference between filters that are both classified by manufacturers as ‘hydrophobic’
Why is the thicker media with greater pore volume less prone to blockage when moisture is present? First, let’s define the ‘moisture’ that causes the problems. High humidity does not necessarily create an issue as this is not in droplet form and passes through the system with the air. If there is a lot of water, perhaps due to heavy rain, this is also not generally a problem as the large water droplets bounce off the filter or drain down the surface of the media.
Mist and fog, however, have droplets that are of a comparable size to the fine particles filtered by HEPA filters and so a mist or fog event is equivalent to a sandstorm of very fine particles causing similar effects in the filter, where the droplets get into the media and clog it. Liquid droplets, once within the matrix of the media, combine and would normally drain out, but when a filter is loaded with hydrocarbon, this changes the surface tension on the media fibres. The water droplets are then more likely to cling to the fibres and remain within the media matrix, causing blinding and a high pressure loss.
Microfibre glass and membrane technology achieve high efficiency filtration in different ways. Microfibre glass media uses its depth to capture particles which have to travel through a tortuous path inside the matrix. Membranes, however, use a thin layer of finer pores which create a ‘sieving’ effect. This means the pores inside the microfibre glass media are larger and small water droplets are more likely to work their way through and, even if they remain within the media, they are less likely to plug the higher volume of larger pores.
As well as selecting an appropriate media type, filters also need to be robust to ensure their reliability. If particulates are allowed to bypass the filter media, its efficiency rating is useless.
Using the right glue, avoiding glue beads that may fall off once a filter is installed, and ensuring that the overall filter design is robust are important considerations. Features such as seamless gaskets on filters also help to prevent leaks during operation. Frame materials should always be selected to handle the installation’s environmental conditions and designed to remain robust after “aging”.
One problem operators face when selecting a filtration solution is that standard efficiency ratings do not necessarily reflect how a filter will perform in the real world. Current ratings are based on laboratory testing that does not cover the wide variety of conditions and environments a turbine may be subjected to.
Take, for example, the hydrophobicity of a filter: its ability to prevent liquid and salts from passing through, causing turbine corrosion and accelerated fouling. This should be a major consideration for the power industry, but there is no industry standard test to rate the performance of a filter in this area. CLARCOR uses a patented hydrophobic test to determine the performance of its filters. It is designed to test areas such as salt leaching and takes the filter through a total of nine wet/dry cycles within a ten-day protocol to simulate filter performance in real-world installations.
A major gas turbine supplier recently carried out comparative tests on the performance of equivalent ePTFE membrane and microfibre glass HEPA products. The tests covered overall efficiency, pressure loss, hydrophobic performance, wet performance and the dust-holding capacity of each technology. Microfibre glass filters showed equal or better performance than ePTFE at a lower cost. The gas turbine supplier then chose the microfibre glass filter as its filter of choice for its next-generation technology.
Overall, the real-world performance of gas turbine filters has shown that, for the moment, microfibre glass technology offers a more predictable and reliable solution than membranes such as ePTFE media. It offers a higher pore volume which is less sensitive to plugging with moisture and hydrocarbons while providing the same dust filtration efficiency.
The predictable nature of microfibre glass filters means they have a much longer life, with a gradual increase in pressure loss over time that is less likely to trip turbines or require unexpected maintenance. They offer a robust, lower-cost, longer-life (H)EPA filter solution.
Although ePTFE membrane filters can offer a typical lifespan of two years, this is much shorter than microfibre glass equivalents. The unpredictable nature of membrane filters further means they require much closer monitoring and any change-out needs to happen quickly to avoid damage to other systems. Overall, they may currently appear to offer a good solution, but operators should take into account maintenance frequency and lifetime costs when considering their selection.
What may the future hold?
Industry experience indicates that microfibre glass technology offers operators greater peace of mind with regard to predictable, reliable turbine output.
For this reason, CLARCOR does not currently recommend the use of ePTFE media on turbine installations. CLARCOR is an in-house manufacturer of ePTFE membrane, uses it successfully in many of its other filtration products, and would like to be able to provide it for GT inlet filtration – but the global diversity in ambient air contaminants makes its performance just too unpredictable today for use with gas turbines.
As membrane technology continues to be developed, however, improvements in its sensitivity to moisture and hydrocarbons may well make it a good option in the future. Other materials such as nanofibres also continue to be developed. These are synthetic polymer-based media very similar to microfibre glass that may soon offer a further viable alternative solution with HEPA filtration efficiency levels.
Testing standards continue to be researched and developed for the implications of different environmental factors on the performance of a filter and, therefore, the performance of turbines is more clearly understood. Hopefully, in the near future, standards will give operators a more comprehensive evaluation of how a filter will operate in the real world.
Steve Hiner is Chief Engineer, Gas Turbine Inlet Systems at CLARCOR Industrial Air. www.clarcorindustrialair.com